N2A, one of the sellers of microSD cards that convert a Nook tablet into a full Android tablet, is now offering a similar solution for the Kindle Fire. It's not nearly as quick and easy (nor as foolproof) as simply inserting a microSD card, but for a mere $20 you can convert a Kindle Fire into a full on Android tablet.
N2A's new N2Aos service will install a customized Android 4.2 Jelly Bean on either the original Kindle Fire or the second-gen model, but not the later KFHD (support for those models is in the works). This version of Android is based on CyanogenMod open source project, and it offers Google Play, a fully open Android tablet, and a suite of useful apps including the reading apps that Amazon doesn't want to let you know exists.
But before you jump at this opportunity to get more use out of your older Kindle Fire, let me add that there's a catch.
First, the process is more complicated than for the Nook hardware. The Kindle Fire lacks a card slot, and that means that the OS will need to be installed from your computer. It's going to wipe whatever is on the Kindle Fire, costing you everything you have downloaded, and there is also a small chance that it might brick your tablet. This process also voids your warranty, obviously.
You'll also have to give up the wonderful ads on the lock screens as well as Amazon Prime Video (here's no Amazon Instant Video app). That last is enough to discourage me, but it might not matter to you.
All in all, I don't think this is a good value if you plan to go out and buy a tablet today. Your total cost is $180, and I think the Nexus 7 at $200 is a better buy. The extra $20 gives you piece of mind and a warranty. But if you already have a Kindle Fire sitting around, the $20 for the DIY service might be worth it. There is also several more expensive options that include paid tech support.
P.S. I think the best value right now is the Nook HD at $129. It's something of a gamble whether B&N will still be supporting it 6 months from now but the odds are good that you won't need a lot of support.